1. Field of the Invention
The invention relates to an inkjet printhead, and particularly to an inkjet printhead having a plurality of pressure chambers, and a common ink chamber from which ink is distributed to the pressure chambers.
2. Description of the Related Art
As disclosed in JP-A-2000-43253 (especially FIG. 1) for instance, there is known an inkjet printhead comprising a cavity unit which is a laminate of a plurality of plates, and a piezoelectric actuator unit superposed on the cavity unit. The plates constituting the cavity unit include a nozzle plate through which a plurality of nozzles are formed in a row, a cavity plate through which a plurality of through-holes constituting a plurality of pressure chambers are formed in a row to positionally correspond to the row of the nozzles, and a manifold plate through which a through-hole constituting a common ink chamber is formed to store ink supplied from an ink supply source and to distribute the ink to the pressure chambers. The piezoelectric actuator unit is disposed on the cavity unit so as to selectively pressurize the ink in the pressure chambers, to eject droplets of the ink from the nozzles as desired. The common ink chamber is formed through the manifold plate to be elongate in the direction of the alignment of the pressure chambers such that the common ink chamber overlaps a part of each pressure chamber in plan view, that is, when seen in the direction of the superposition of the cavity and actuator units. The ink is introduced into the common ink chamber at an ink introduction place located at a first one of two opposite longitudinal ends of the common ink chamber, and flows toward a second or the other longitudinal end, as the ink is distributed to the pressure chambers sequentially.
In such a printhead, when the width of the common ink chamber is constant throughout the entire length thereof, there is a high possibility of failure in ejection of ink droplets, that is, ink droplets may not be properly ejected as intended, or may be completely failed to be ejected. This is because that as the ink introduced at the ink introduction place flows in the common ink chamber toward the second longitudinal end far from the ink introduction place, the ink flow decelerates, and stagnation of the ink flow tends to occur at the second longitudinal end, causing accumulation or stay of air bubbles there which leads to the failure in ejection of ink droplets.
In order to prevent the stagnation of the ink and solve the problem with the accumulation of air bubbles, the above-mentioned publication discloses to narrow the common ink chamber at the second longitudinal end so as to gradually reduce the cross-sectional area of the common ink chamber perpendicular to the longitudinal direction of the common ink chamber.
Meanwhile, all the pressure chambers have a same length. Thus, in the manifold plate, the ratio of an open area (corresponding to the through-hole of the common ink chamber) to a non-open area (corresponding to the material or substance forming the manifold plate and present around the common ink chamber) at a place corresponding to each of the pressure chambers varies among the pressure chambers. In other words, the ratio of the open area to the non-open area varies along the longitudinal direction of the common ink chamber This produces a variation in the rigidity of a member or members surrounding the pressure chambers aligned along the longitudinal direction of the common ink chamber. Hence, the characteristic period of propagation of a pressure wave in the ink through an ink channel which is partially constituted by each pressure chamber varies among the pressure chambers. That is, the characteristic period of pressure wave propagation is a period of time taken for a pressure wave generated in the ink in a pressure chamber when the piezoelectric actuator unit is driven, to propagate to and back from the nozzle, or to propagate both ways with respect to the longitudinal direction of the pressure chamber. The characteristic period is a function of the length of the pressure chamber, the sound speed in the ink, and the rigidity of the member(s) surrounding the pressure chamber or the characteristic period. Since an ink droplet is ejected through a nozzle based on the pressure wave, or by superposing the next pressure wave on a pressure wave, the two pressure chambers at the opposite ends of the row of the pressure chambers and located over the opposite ends of the common ink chamber exhibit a difference in their ink ejection characteristics, such as the speed of the ejected ink droplet and the stability in the ink ejection, for a same drive signal.
In view of this problem, the present applicant has proposed, in JP-A-2002-137386 (especially FIG. 9), to compensate for the variation in a supporting rigidity of the manifold plate with respect to the pressure chambers, along the longitudinal direction of the common ink chamber, by forming an empty chamber at the side of the narrowed portion of the common ink chamber such that the sum of the cross-sectional area of the common ink chamber and that of the empty chamber as taken in the direction perpendicular to the longitudinal direction of the common ink chamber is substantially identical at every position in the longitudinal direction. Thus, it is designed to uniform the supporting rigidity of the manifold plate with respect to the pressure chambers across a region corresponding to the row of the pressure chambers.
With such an empty chamber, this conventional technique succeeds in substantially reducing the variation in the rigidity along the direction of the row of the pressure chambers. However, the empty chamber and the common ink chamber in which air and the ink are accommodated, respectively, are provided by through-holes formed through the manifold plate which is of metal, with the metal material of the manifold plate present between the empty chamber and the common ink chamber. Thus, the common ink chamber is surrounded, along the entire circumference thereof inclusive of its narrowed portion, by the highly rigid metal material, resulting in a difference in the speed of the one-way propagation of pressure in the ink, between the pressure chamber corresponding to the narrowed portion under influence of the presence of the metal material between the empty chamber and common ink chamber, and the pressure chamber corresponding to the other portion free from such influence. Thus, this technique fails to eliminate the variation in the ink ejection characteristics completely, and a further improvement has been desired in this regard.